1. Field of the Invention
This invention relates to an FM multiplex demodulator circuit. More particularly, it is directed to an FM multiplex demodulator circuit which is constructed into a semiconductor integrated circuit.
2. Description of the Prior Art
Although the PLL(phase locked loop) technique is conventional, because of the need for a complicated circuit arrangement and a high cost, its fields of utilization have been restricted to measuring instruments, space communications, etc.
With the progress and development of semiconductor integrated circuit technology, the fields of utilization have been expanded.
As an example, a double balance type synchronous detection circuit employed in an FM multiplex demodulator circuit has been developed. This circuit and a semiconductor integrated circuit are inseparable. The FM multiplex demodulator circuit formed as a semiconductor integrated circuit has heretofore acquired signal outputs of a right channel and a left channel by passing outputs of the synchronous detection circuit through an LPF (low-pass filter) as an externally-mounted circuit.
In a differential circuit which constitutes the synchronous detection circuit, in order to make the number of externally-mounted pins small and to realize de-emphasis, a load resistance determining the gain of the differential circuit has been formed by an externally-mounted resistance and it has been impossible to make the value of the resistance large.
Therefore, the following problems are involved:
1. The obtainable detection output is only several tens of mV-500 mV or so. Since the detection output is small, an amplifier circuit is required separately from the FM multiplex demodulator circuit constructed into the semiconductor integrated circuit device.
2. Since the load of the detection circuit is the externally-mounted resistance, the gain of this circuit is determined by the ratio between the externally-mounted resistance and an internal resistance (a resistance formed within the semiconductor integrated circuit). The dispersion of the values of the internal resistances is as large as .+-.30 to 50% or so with respect to a design value, so that the gains are largely dispersed.
3. The LPF (low-pass filter) has a low input impedance, so that its impedance matching with the detection circuit cannot be established. Where the LPF and the detection circuit are directly coupled, the intrinsic characteristic of the LPF cannot be attained.